Secondary-port bar crossing: latest ebb
You are skippering a 10.5 m yacht with draft 1.9 m. You plan to cross a drying bar and enter a marina. The charted depth on the bar at chart datum is 1.1 m. You require at least 0.6 m under the keel. The standard port tide table gives LW 0530 height 0.8 m and HW 1130 height 4.6 m. The secondary port differences are LW time +0005, LW height +0.1 m; HW time −0010, HW height −0.2 m. It is 1400. The bar passage takes 20 minutes and the shallowest point is 10 minutes after entry. You want to be inside the marina before 1700. Wind is NW 15 kn, sea slight, but the bar is exposed to swell. The engine is reliable and you have an echo sounder, paper chart and tide table. Crew of four, one nervous. There is no safe alternative anchorage within 5 nm.
1. Calculate the secondary-port LW and HW, the tidal range, and the minimum height of tide you need over the bar.
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Apply the differences to the standard port. LW at the secondary port = 0530 + 0005 = 0535, height 0.8 + 0.1 = 0.9 m. HW = 1130 − 0010 = 1120, height 4.6 − 0.2 = 4.4 m. Range = 4.4 − 0.9 = 3.5 m.
Required tide height = draft + under-keel clearance − charted depth = 1.9 + 0.6 − 1.1 = 1.4 m. At LW the height is 0.9 m, so you need 1.4 − 0.9 = 0.5 m of rise above LW.
2. Using the rule of twelfths, determine the latest time you should be over the shallowest point on the ebb, and the latest time to start the crossing. Show the twelfths.
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Range 3.5 m, so one twelfth = 0.292 m. On the ebb, height above LW is: 1 h before LW = 1/12 = 0.292 m; 2 h before LW = 3/12 = 0.875 m; 3 h before LW = 6/12 = 1.75 m.
You need 0.5 m above LW, which lies between LW−2 h and LW−1 h. Interpolate: (0.875 − 0.500)/(0.875 − 0.292) = 0.375/0.583 = 0.643 of the hour, i.e. 38.6 min after LW−2 h. That is 1 h 21.4 min before LW. Secondary LW is 1735, so the latest time over the shallowest point is about 1614 (say 1610 to be conservative).
The shallowest point is 10 minutes after entry, so start the crossing by about 1604 (say 1600). That still allows entry before 1700.
3. State two reasons the rule-of-twelfths result may be unsafe near low water and what you would do.
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The rule assumes a simple sinusoidal tide. In the first and last hours it can overestimate the height of tide compared with a real tidal curve, which is unsafe when you are relying on a small margin above chart datum. Real depth is also affected by barometric pressure, wind, swell and river flow, and the charted bar depth may have changed.
Use the actual tidal curve or port authority information if available, add a bigger margin, keep the echo sounder running, reduce speed, and be prepared to turn back. Do not treat the twelfths answer as a guarantee of depth.